The role of FSH in spermatogenesis and male fertility remains unclear. Hypophysectomy and hormone replacement therapy have been the classical methods of examining the influence of hormones on gonadal function. However, with these techniques it has not proved possible to dissect the respective roles of LH and FSH. A more elegant experimental tool to study this topic is through the use of highly specific antigonadotropic sera in passive immunization experiments. We plan to prepare highly purified antiserum to rat FSH and inject it into adult male rats for long periods of time (up to 60 days). Our objective is to determine whether selective reduction of this gonadotropin leads to impaired spermatogenesis and fertility. Morphological and biochemical parameters will be examined following antisera treatment.